Grant was demolished by small and skinny Guinn and schooled by LHW Adamek, Rahman struggled badly against old, fat middleweight James Toney, Briggs was KTFOed by 5'10'' Darnell Wilson, Tucker lost to Marciano-sized Norris etc. Yes, if Marciano was fighting them all in a row, the toll from slugging with big guys would make a serious impact on his body and he would probably become washed up at 29-30. But I'd favor the best version of Rocky against most of them
Thinking over it again yes I believe Rocky knocks out Briggs, obviously Briggs loves to start fast so Rocky better be ready though but Briggs has terrible stamina and Rocky has great stamina. I did mention I favour Rocky to knock out Botha, Weaver, McCall and Tua (Tua being the most dangerous). Rahman didn't look great against Toney, but Toney is very awkward to look good against and was aot heavier than Marciano, Rahman still should have gotten the victory. I also wouldn't say that's the best version of Rahman at all to compare and I'd pick Marciano to beat Toney. Michael Grant had a glass jaw but he was still 6'7, 250+ pounds and had an 86 inch reach compared to the 5'10 185 pound Marciano who had a 67 inch reach. Has any boxer ever won a fight with 19 inch reach disadvantage, 9 inch disadvantage in height and 65+ pound disadvantage? Grant showed plenty of heart against Andrew Golota too, again Grant is far from the best heavyweights but with those physical advantages I'm sure he could be trained to beat someone that much smaller.
Chagaev vs Valuev: 11 inch disadvantage in height, 101+ disadvantage in weight and 11 inch disadvantage in reach
Nice, Grant was a lot more athletic and faster than Valuev though. I'm just saying it is possible that an average modern day SHW could have beaten Marciano.
Lewis outweighed Marciano by 60 pounds and hits alot harder, Briggs also rocked Lewis early on and gave a good account of himself. Briggs literally got up from the most picture perfect right hand i've ever seen in the 5th round, and his actual stoppage vs Lewis was from him missing a punch and falling over. Briggs is also alot bigger Marciano and would by far be the biggest puncher Marciano ever fought who is an actual Heavyweight by modern standards.